The present invention relates to capacitive touch switches and in particular to a capacitive touch switch having improved immunity against false triggering.
Touch switches, which sense a change in the electrical environment around the touch point, for example, when a finger is moved over a stationary electrode near the touch point, provide a number of advantages over conventional mechanical switches with movable contacts. Because touch switches have no moving parts, they are particularly well-suited to applications where there is mechanical shock and long life is required. Further, because touch switches do not require a movable operator, they may be easily sealed from environmental contaminants such as dirt or water.
One type of capacitive touch switch senses a change in capacitive coupling between an antenna electrode and a sense electrode. This change may be, for example, a decrease in coupling between the antenna and sense electrode caused by the diversion of electrical energy into a capacitive coupling to the user's hand. By sensing capacitive effects only, it is possible to cover the sensing electrodes with an insulating protective layer.
For capacitive touch switches of reasonable size, the measured capacitive coupling is relatively small and thus sensitive circuitry must be used to detect the “touch”. Such sensitive circuitry is prone to false triggering caused by electromagnetic interference from other electrical devices. In addition, the small changes in capacitive coupling caused by a finger touch, can often be overwhelmed by larger environmental capacitive changes, for example, those caused by environmental contaminants such as dirt, water, or ice, preventing detection of the finger touch.
While the mechanical advantages of capacitive touch switches recommend them for automotive use where they would resist mechanical shock and environmental contamination, the problems of false triggering and signal saturation have prevented their widespread adoption.